This application relates in general to a, method, apparatus, and article of manufacture for providing high speed digital communications through a communications channel, and more particularly to a method, apparatus, and article of manufacture for encoding and decoding digital communications data.
Digital communications channels are continuously increasing the transfer rate at which the data is transmitted between devices in order to meet the demand on communications systems. To meet that increase, conventional systems have employed various encoding and modulations techniques that attempt to meet that demand. One such technique is to encode user data into what are known as “Q Codes”.
In order to further meet the increase in the demand of the transfer rate of data, conventional system also employ a multi-level pulse amplitude modulation (PAM-n) scheme that encodes a plurality of bits of data into or onto a signal in which the symbols are represented as a set of signal levels between a minimum and maximum set of signal levels. One advantage of utilizing multi-level PAM, as opposed to binary, is that fewer symbols per second need to be transmitted to convey a given number of bits per second. This reduces the frequency content of the transmitted signal.
A shortcoming of employing multi-level PAM signaling is a reduction in voltage margin. The reduction in voltage margin creates, among other things, the recovery or detection of the original signal more difficult and therefore recovery of the original symbol(s) more difficult. For example, detecting the level of a PAM-4 signaling technique requires discrimination between four levels rather than two levels in a binary signaling technique.
Moreover, while PAM-4 signaling allows for a two-fold reduction in the symbol rate for a given channel bit rate (relative to binary), systems employing PAM-4 tend to observe or experience a two-thirds smaller eye opening in terms of the voltage, and up to a two times larger eye opening in terms of the time domain relative to conventional binary signaling systems carrying the same aggregate bit rate. As such, nearly exact or precise equalization to a zero intersymbol interference pulse shape may be required to eliminate errors.
Further, systems employing PAM encoding techniques, for example, PAM-4, may experience signal transitions at the transmitter that traverse the spectrum of allowable signal levels. That is, a transition from the highest to lowest allowable signal levels, for example, transition from a signal level of +3 to a signal level of −3, may enhance the difficulty in recovery of the signals at the receiver because such transitions tend to narrow the “eye” openings from the perspective of the receiver.
In short, the present invention addresses, among other things, some or all of the shortcomings of communication systems employing PAM-n (for example, PAM-4) signaling techniques.